U.S. patent number 6,828,556 [Application Number 10/256,335] was granted by the patent office on 2004-12-07 for millimeter wave imaging array.
This patent grant is currently assigned to HRL Laboratories, LLC. Invention is credited to Peter D. Brewer, Mehran Matloubian, Carl W. Pobanz.
United States Patent |
6,828,556 |
Pobanz , et al. |
December 7, 2004 |
Millimeter wave imaging array
Abstract
A focal plane array for millimeter wave imaging comprising a
three dimensional stack of antenna elements and radiometer
microwave monolithic integrated circuits (MMICs) embedded in
polymer dielectric layers built on top of a silicon substrate. Each
radiometer MMIC and antenna element comprise a radiometer pixel.
The silicon substrate contains integrated circuits to collect and
process the signals from each radiometer pixel and generate a
full-frame video signal. The array can be fabricated on a single
silicon wafer or can be constructed from structures fabricated on
multiple silicon wafers.
Inventors: |
Pobanz; Carl W. (Rancho Palos
Verdes, CA), Matloubian; Mehran (Encino, CA), Brewer;
Peter D. (Westlake Village, CA) |
Assignee: |
HRL Laboratories, LLC (Malibu,
CA)
|
Family
ID: |
27400944 |
Appl.
No.: |
10/256,335 |
Filed: |
September 26, 2002 |
Current U.S.
Class: |
250/336.1;
257/E27.136; 343/700MS |
Current CPC
Class: |
H01L
27/14649 (20130101) |
Current International
Class: |
G02F
1/01 (20060101); G01T 1/00 (20060101); H01L
31/00 (20060101); H01Q 21/00 (20060101); H01Q
021/00 () |
Field of
Search: |
;250/336.1
;343/700MS |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Kenjiro Nishikawa et al., "Three-Dimensional MMIC Technology for
Low-Cost Millimeter-Wave MMICs." Sep., 2001. IEEE Journal of
Solid-State Circuits, vol. 36, No. 9, pp. 1351-1359.* .
Kenjiro Nishikawa et al., "Miniaturized Millimeter-Wave Masterslice
3-D MMIC Amplifier and Mixer." Sep., 1999. IEEE Transactions on
Microwave Theory and Techniques, vol. 47, No. 9, pp. 1856-1862.*
.
Ichihiko Toyoda et al., "Highly Integrated Three-Dimensional MMIC
Single-Chip Receiver and Transmitter." Dec., 1996. IEEE
Transactions on Microwave Theory and Techniques, Vo. 44, No. 12,
pp. 2340-2346.* .
Kuroda, R.T., et al, "Large Scale W-Band Focal Plane Array
Developments for Passive Millimeter Wave Imaging," Passive
Millimeter-Wave Imaging Technology II, Proceedings of the SPIE--The
International Society of Optical Engineering, vol. 3378, pp. 57-62
(Apr. 1998). .
Ferendeci, A. M., "Monolithically Processed Vertically
Interconnected 3D Phased Array Antenna Module," IEEE Proceedings,
National Aerospace and Electronics Conference 2000, pp 153-155
(Oct. 12, 2000). .
Seki, T., et al., "Active Antenna Using Multi-Layer
Ceramic-Polyimide Substrates for Wireless Communication System,"
Microwave Symposium Digest, 2001 IEEE MTT-S International, pp
385-388 (May 25, 2001.)..
|
Primary Examiner: Hannaher; Constantine
Attorney, Agent or Firm: Ladas & Parry
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
The present document is related to and claims the benefit of the
copending and commonly assigned patent application document
entitled: "Millimeter Wave Imaging Array," Serial No. 60/326,053,
filed on Sep. 28, 2001. The contents of this application is hereby
incorporated by reference herein. The present document is also
related to the following copending and commonly assigned patent
application documents: "Process for Assembling Three-Dimensional
Systems on a Chip and Structure Thus Obtained," Serial No.
60/326,076; "Process For Producing High Performance Interconnects,"
Serial No. 60/326,054; "Method For Assembly Of Complementary-Shaped
Receptacle Site And Device Microstructures," Serial No. 60/326,055;
and "Method of Self-Latching for Adhesion During Self-Assembly of
Electronic or Optics Circuits," Serial No. 60/326,056, all of which
were filed on Sep. 28, 2001. The contents of these related
applications are hereby incorporated by reference herein.
Claims
What is claimed is:
1. An apparatus for millimeter wave imaging comprising: a substrate
comprising a first plurality of integrated circuits; one or more
receptacle layers positioned on top of the substrate, each
receptacle layer of the one or more receptacle layers comprising: a
second plurality or integrated circuits encapsulated in a polymer
film, a plurality of vertical interconnect; and an antenna carrying
layer comprising: an antenna dielectric layer and a plurality of
antenna elements positioned on top of the antenna dielectric
layer.
2. The apparatus according to claim 1 further comprising: one or
more first encapsulation layers, each encapsulation layer located
above each receptacle layer of said one or more receptacle layers
and at least one encapsulation layer of the one or more
encapsulation layers comprising: a dielectric layer and a plurality
of horizontal interconnects deposited on top of the dielectric
layer, the plurality of horizontal interconnects connecting the
vertical interconnects, the first plurality of integrated circuits,
and the second plurality of integrated circuits.
3. The apparatus according to claim 1 further comprising: a metal
layer deposited beneath the antenna dielectric layer; and a ground
plane dielectric layer located beneath the metal layer.
4. The apparatus according to claim 1 wherein said plurality of
antenna elements are arranged such that each antenna element in
said plurality of antenna elements is separated from another
antenna element in two perpendicular directions.
5. The apparatus according to claim 1 wherein each integrated
circuit of the second plurality of integrated circuits comprises a
radiometer having: a radio frequency input connected to a single
antenna element of said plurality of antenna elements; a control
input; a power input; and, a video output;
and wherein each integrated circuit of the first plurality of
integrated circuits comprises a radiometer processor having: a
control output connected to the control input of said radiometer; a
power output connected to the power input of said radiometer; a
video input connected to the video output of said radiometer; a
digital video output; a direct current input; and, a clock
input.
6. The apparatus according to claim 5 wherein the radiometer
comprises: a temperature reference; a radio frequency switch having
a first input connected to the radio frequency inputs, a second
input connected to the temperature reference, and a switch output,
the radio frequency switch connecting the first input or the second
input to the switch output, and the radio frequency switch
controlled by the control input; a low noise amplifier having an
amplifier input and an amplifier output, the amplifier input
connected to the switch output; and a power detector having a
detector input and a detector output, the detector input connected
to the amplifier output, and the detector output connected to the
video output.
7. The apparatus according to claim 5 wherein the radiometer
processor comprises: a direct current voltage regulator providing
regulated power to the power output; a chopping signal generator
with chopping output, the chopping output connected to the control
output; a synchronous video detector connected to the video input
and to the chopping output and providing a detected video output;
an integrator connected to the detected video output and providing
an integrated video output; and an analog-to-digital convener
connected to the integrated video output and providing the digital
video output.
8. The apparatus according to claim 5 wherein said radiometer
comprises a microwave monolithic integrated circuit.
9. The apparatus according to claim 8 wherein said microwave
monolithic integrated circuit comprises a three dimensional
multiple layer structure, the three dimensional multiple layer
structure comprising: an RF MEMS switch in a first layer, the RF
MEMS switch having a first input connected to the radio frequency
input, a second input connected to a temperature reference, and an
RF switch output, said RF MEMS switch being controlled by said
control input; a low noise amplifier integrated circuit in a second
layer, said low noise amplifier integrated circuit having an
amplifier input and an amplifier output, said amplifier input being
connected to said RF switch output; and a power detector integrated
circuit in a third layer, said power detector integrated circuit
having a power detector input connected to said amplifier output
and having a power detector output connected to said video
output.
10. The apparatus according to claim 9 wherein said low noise
amplifier integrated circuit comprises an HEMT built on an InP
substrate.
11. The apparatus according to claim 9 wherein said power detector
integrated circuit comprises a backward diode.
12. A millimeter wave focal plane array comprising: one or more
focal plane array structures comprising: a semiconductor wafer; a
first plurality of integrated circuits formed within the
semiconductor wafer; one or more receptacle layers positioned on
top of the semiconductor wafer, each receptacle layer comprising a
second plurality of integrated circuits encapsulated in a polymer
film; and an antenna carrying layer positioned on top of the
receptacle layer, the antenna carrying layer comprising: an antenna
dielectric layer and a plurality of antenna elements positioned on
top of the antenna dielectric layer.
13. The millimeter wave focal plane array according to claim 12
wherein at least one integrated circuit in the second plurality of
integrated circuits comprises a radiometer and at least one
integrated circuit in the first plurality of integrated circuits
comprises a radiometer processor connected to said radiometer.
14. The millimeter wave focal plane array according to claim 13
further comprising: a video frame processor connected to every
radiometer processor.
15. The millimeter wave focal plane array according to claim 13
wherein said radiometer comprises a three dimensional multiple
layer structure, the three dimensional multiple layer structure
comprising: an RF MEMS switch in a first layer, the RF MEMS switch
having a first input connected to one antenna element of the
plurality of antenna elements, a second input connected to a
temperature reference, and an RF switch output; a low noise
amplifier integrated circuit in a second layer, said low noise
amplifier integrated circuit having an amplifier input and an
amplifier output, said amplifier input being connected to said RF
switch output; and a power detector integrated circuit in a third
layer, said power detector integrated circuit having a power
detector input connected to said amplifier output and having a
power detector output.
16. The millimeter wave focal plane array according to claim 15
wherein said low noise amplifier integrated circuit comprises an
HEMT built on an InP substrate.
17. The millimeter wave focal plane array according to claim 15
wherein said power detector integrated circuit comprises a backward
diode.
18. A millimeter wave imaging system for producing a display based
on millimeter wave radiation received from a viewed scene, said
system comprising: a millimeter wave focal plane array producing
full image video signal, said array comprising a substrate
containing a plurality of substrate integrated circuits; one or
more receptacle layers, each receptacle layer comprising a
plurality of radiometer integrated circuits encapsulated in a
polymer film; and an antenna carrying layer having a plurality of
antenna elements; and a lens directing the viewed scene onto the
millimeter wave focal plane array; a processing unit receiving the
full image video signal and producing a display video signal; and a
video display generating a visual display from the display video
signal.
19. The millimeter wave imaging system according to claim 18
wherein said substrate comprises a single silicon wafer.
20. The millimeter wave imaging system according to claim 18
wherein said substrate comprises multiple silicon wafers.
21. The millimeter wave imaging system according to claim 18
wherein one substrate integrated circuit of said plurality of
substrate integrated circuits comprises circuitry to generate a
full frame video signal and each remaining substrate integrated
circuits of plurality of substrate integrated circuits comprises a
radiometer processor.
22. The millimeter wave imaging system according to claim 18
wherein each radiometer integrated circuit of said plurality of
radiometer integrated circuits comprises a three dimensional
multiple layer structure, the three dimensional multiple layer
structure comprising: an RF MEMS switch in a first layer, the RF
MEMS switch having a first input connected to one antenna element
of the plurality of antenna elements, a second input connected to a
temperature reference, and an RF switch output; a low noise
amplifier integrated circuit in a second layer, said low noise
amplifier integrated circuit having an amplifier input and an
amplifier output, said amplifier input being connected to said RF
switch output; and a power detector integrated circuit in a third
layer, said power detector integrated circuit having a power
detector input connected to said amplifier output and having a
power detector output.
23. The millimeter wave imaging system according to claim 18
wherein the system is contained within a single handheld
enclosure.
24. The millimeter wave imaging system according to claim 18
wherein the lens is a zoom lens.
25. The millimeter wave imaging system according to claim 18
wherein the video display comprises a liquid crystal display.
Description
BACKGROUND
1. Field
This invention relates generally to millimeter wave imaging systems
and, more particularly, to a millimeter wave imaging system
including a highly-integrated millimeter wave focal plane
radiometer array.
2. Description of Related Art
Generation of images responsive to detected millimeter waves
(radiation having wavelengths in approximately the 1 cm-1 mm range,
that is, frequencies between 30 GHz and 300 GHz) reflected from or
emitted by objects in a field of view is desired in many
applications. This is largely because millimeter waves penetrate
many materials that are opaque to visible and infrared radiation,
enabling high-resolution imaging of scenes that were previously
invisible. For example, millimeter wave imagers could provide
landing assistance to aircraft for runways obscured by fog.
Additionally, millimeter wave imagers could provide images of
weapons concealed beneath clothing, since human bodies and metal
objects have different optical properties at millimeter
wavelengths.
Since all objects reflect and emit millimeter waves, passive
imaging can be used to detect the natural millimeter wave emissions
or reflections from objects or people. The emissivity range of
objects at millimeter waves is very large, approximately ten times
greater than the range provided by infrared, so high contrast
images can be made using existing blackbody radiation. A passive
imager uses sensitive receivers to distinguish small differences in
millimeter wave emissions. The emitted radiation is processed by
the detector which converts the millimeter wave emissions down to a
video signal. The strength of the video signal is roughly
proportional to the power level in the emitted radiation.
Creating an image from emitted millimeter wave radiation has been
historically difficult due to the lack of small, sensitive
millimeter wave detectors that can be easily arrayed. Early
versions of millimeter wave imaging systems used mechanical or
electronic scanning of the millimeter wave sensor. Mechanical
systems physically move a sensor through a range of azimuths,
elevations, or both, defining a field of view. Such systems are
complex and subject to failure. Electronic scanning typically
requires employment of electronic phase shifting or switching
techniques which are relatively complex to implement at millimeter
wave frequencies.
Later generations of millimeter wave imaging systems used focal
plane arrays of millimeter wave detectors. These systems are
characterized by the use of conventional two dimensional
integration of electronics using circuit board techniques. U.S.
Pat. No. 4,910,523 issued Mar. 20, 1990 to R. G. Huguenin, et al,
discloses a focal plane array comprising multiple circuit boards
disposed in a horizontal direction where each circuit board has
multiple detectors disposed in a vertical direction. U.S. Pat. No.
5,438,336 issued Aug. 1, 1995 to P. S. C. Lee, et al, also
discloses a focal plane array for millimeter wave imaging. In Lee,
an array of pixels are used for detection of millimeter wave
images. Each pixel comprises an antenna, a low noise amplifier, a
band pass filter and a video detector. However, Lee discloses using
discrete parts for these elements, so the level of integration
occurs at the level of a single pixel, and the pixels would be
constructed using circuit board techniques. Lee also discloses that
signal processing of the detected signal is done externally to the
individual pixels.
A millimeter wave video camera operating at 89 GHz is disclosed by
R. T. Kurado et al., in "Large Scale W-Band Focal Plane Array
Developments for Passive Millimeter Wave Imaging," SPIE Conference
on Passive Millimeter-Wave Imaging Technology, April 1998, pp.
57-62. The millimeter wave imager disclosed by Kurado is
approximately 30 inches on a side and is assembled using hybrid
circuit-card techniques. The focal plane detector within the
disclosed imager uses 1040 MMICs and antennas, along with 15,860
resistors, capacitors, and silicon integrated circuits
interconnected by 36,920 wire bonds.
U.S. Pat. No. 5,237,334 issued Aug. 17, 1993 to W. M. Waters
discloses a focal plane antenna array comprising a plurality of
conical horns and circular waveguides. The waveguide array is
constructed by perforating an aluminum plate to provide passages
defining the waveguides. Diodes are then manually assembled into
each aperture to provide detection of millimeter waves. Signal
processing of the detected millimeter waves is done by a separate
signal processing module. Conventional wiring leads are used to
connect the array of detectors to the signal processing module.
All of the systems discussed above use conventional circuit board
integration where components are serially picked-and-placed into a
primarily two dimensional structure. These systems suffer from a
large size and time-consuming serial assembly resulting from the
use of conventional two dimensional module-style integration to
implement a dense, inherently three-dimensional array. Such systems
also exhibit degraded system performance due to unnecessarily long
interconnections between electronic components. Construction of
large, costly detector elements may force the use of
sparsely-populated arrays with image resolution that falls well
short of theoretical limits. Attempts to save space by sharing and
multiplexing one video processor among a number of detector
elements, or by using mechanically scanned optics, results in slow
refresh rates that are inadequate for real-time video. These
limitations have prevented widespread deployment of millimeter wave
imaging technology.
In light of the discussion above, there exists a need in the art
for a compact focal plane array for millimeter wave imaging.
Specifically, such an array should maximize integration of the
elements required for millimeter wave imaging while minimizing
reliance on conventional circuit board assembly techniques. Such an
array should also allow for minimum spacing of array elements to
achieve maximum resolution as well as minimizing circuit
interconnection lengths.
SUMMARY
It is an object of the present invention to provide a compact focal
plane array for millimeter wave imaging that can be assembled with
a minimal reliance on conventional circuit board assembly
techniques. It is a further object of the present invention to
allow elements of a millimeter wave focal plane array to be spaced
to achieve maximum resolution of an image processed by the array.
It is another object of the present invention to provide the
capability for real-time video based on detected millimeter wave
radiation.
The present invention comprises a three-dimensional, integrated
focal plane radiometer array structure for millimeter wave imaging.
The structure comprises one or more layers of polymer films
containing encapsulated semiconductor devices, transmission lines,
and circuit interconnects which are positioned on top of a
substrate. The substrate contains multiple integrated circuits
interconnected and connected to the semiconductor devices in the
layers above. The top layer of the structure contain antenna
elements which have been disposed upon a dielectric. The antenna
elements are connected to the semiconductor devices in the layers
below. The small size of the semiconductor devices and the
integrated circuits allows the antenna elements to be closely
spaced and minimizes the interconnection lengths between the
antenna elements, the semiconductor devices, and the integrated
circuits. The small spacing of antenna elements allows the focal
plane array to produce a very high resolution image. Minimization
of interconnection lengths provides that system noise is reduced
and allows a real-time update rate to be used in imaging a
scene.
A first embodiment of the present invention is provided by a
substrate comprising a plurality of integrated circuits; one or
more receptacle layers positioned on top of the substrate, the
receptacle layers each comprising a plurality of microwave
monolithic integrated circuits encapsulated in a polymer film, the
plurality of the microwave monolithic integrated circuits
connecting to the plurality of the integrated circuits by vertical
interconnects; and an antenna carrying layer comprising: a
dielectric layer and a plurality of antenna elements positioned on
top of the dielectric layer, the plurality of antenna elements
connecting to the plurality of the microwave monolithic circuits by
vertical interconnects through the dielectric.
A second embodiment of the present invention is provided by a focal
plane array structure comprising: a semiconductor wafer; a
plurality of integrated circuits formed within the semiconductor
wafer; one or more receptacle layers positioned on top of the
silicon wafer, each receptacle layer comprising a plurality of
microwave monolithic integrated circuits encapsulated in a polymer
film; and an antenna carrying layer positioned on top of the
receptacle layer, the antenna carrying layer comprising: a
dielectric layer and a plurality of antenna elements positioned on
top of the dielectric layer.
The small size of the microwave imaging array provided by the
present invention allows for the deployment of a complete
millimeter wave imaging system within a small, preferably handheld,
enclosure. Such a millimeter wave imaging system comprises: a
millimeter wave focal plane array producing full image video, said
array comprising: a substrate containing a plurality of integrated
circuits; one or more receptacle layers, each receptacle layer
comprising a plurality of integrated circuits encapsulated in a
polymer film; and an antenna carrying layer; and a lens directing
the viewed scene onto the focal plane array; a processing unit
receiving the full image video and producing a display video
signal; and a video display creating a visual display from the
display video signal.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a simplified representation of a millimeter wave focal
plane array provided by the present invention and an exploded view
of two pixels within the array.
FIG. 2 shows a physical representation of the various layers used
within a single radiometer pixel of the present invention.
FIG. 3 shows a block diagram of the electronic elements used in one
embodiment of a radiometer pixel of the present invention.
FIG. 4 illustrates a block diagram for a millimeter wave imaging
system according to the present invention.
FIG. 5 shows a simplified representation of a three dimensional
multiple layer structure providing a microwave monolithic
integrated circuit for use as a radiometer in the present
invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Referring now to FIG. 1, a simplified representation of the
millimeter wave focal plane array in accordance with the present
invention is shown. Millimeter radiation from a scene 30 is
directed onto a focal plane array 10 by Gaussian optics 20 or other
millimeter wave focusing devices. Millimeter wave focusing devices
include refractive optical devices made of plastic lens and
reflective optical devices in a Cassegrain configuration. Other
focusing devices known in the art may also be used.
The focal plane array 10 includes a plurality of radiometer pixels
100. Each radiometer pixel 100 has a size on the order of a single
millimeter wave length, so each radiometer pixel 100 may have an
area from around 1 mm.sup.2 to 1 cm.sup.2. If the focal plane array
10 is fabricated from a single silicon wafer (as described below),
then the focal plane array 10 may comprise from around 100 to
10,000 or more radiometer pixels 100.
The exploded portion of FIG. 1 shows two adjacent radiometer pixels
100 along with an integrated circuit 115 that combines the outputs
from multiple radiometer pixels. The plurality of radiometer pixels
100 comprises a layered three-dimensional structure with one layer
of antennas 131, one or more layers of polymer dielectric film 120
with millimeter wave radiometer microwave monolithic integrated
circuits (MMICs) 121 embedded within, and a final layer comprising
a substrate layer 110 that serves as the base for the structure. In
addition to serving as a foundation for the array, the substrate
layer 110 preferably contains radiometer processor integrated
circuits 111 to collect and process the signals from each
radiometer MMIC 121. Connections between the antennas 131 and the
radiometer MMICs 121 are preferably made by high frequency vertical
interconnects (vias) 122, while connections between the radiometer
MMICs 121 and the integrated circuits are preferably made by low
frequency vias 112. The substrate layer 110 may also contain
additional circuitry 115 to collect and process the signals from
each integrated circuit 111 and to generate a full-frame video
signal. The combination of a single antenna 131, a radiometer MMIC
121, and a radiometer processor integrated circuit 111 forms a
single radiometer pixel 100. FIG. 1 illustrates the structure of
two such radiometer pixels within the millimeter wave focal plane
array 10.
FIG. 2 illustrates the physical relationship of the various layers
of one embodiment of the present invention. FIG. 2 shows the
substrate layer 110, the receptacle layer 120, encapsulation layers
140, and a top antenna carrying layer 130. These layers are bonded
together so as to provide an integral three-dimensional structure.
The substrate comprises silicon or another material such as GaAs or
another group III-V semiconductor in which integrated circuits are
formed. A silicon substrate is preferable since it is low cost and
offers very high levels of integration for circuits formed within
it. The substrate layer 110 contains multiple radiometer processor
integrated circuits 111 for processing millimeter wave video
signals. The integrated circuits 111 use Si CMOS or SiGe CMOS
technology or other technologies appropriate for integrated
circuits formed within the substrate layer 110. Along with
containing integrated circuits 111, the substrate layer 110 forms
the host or base upon which the receptacle layer 120 is registered.
As a base structure, a silicon substrate is also preferable due to
its structural sturdiness.
The receptacle layer 120 comprises a thermoplastic polymer
dielectric chosen for its specific radio frequency characteristics
and for its forming temperature. The receptacle layer 120 receives
the radiometer MMICs 121 used for processing received RF signals.
The receptacle layer 120 is preferably micro-stamped with
specially-shaped receptacles prior to the receipt of the MMICs 121,
so as to allow the MMICs 121 with corresponding specially-shaped
locating features to be positioned and retained within the
receptacle layer 120. These MMICs 121 can be fully fabricated and
tested before they are placed into the receptacle layer 120,
providing for improved yields for focal plane arrays fabricated
using this technology. The MMICs 121 are then encapsulated by an
encapsulation layer 140 as described below. The receptacle layer
120 is bonded to the substrate layer110. Exemplary processes for
preparing the receptacle layer 120 for receipt of the MMICs 121,
transfer and encapsulation of the MMICs 121, and bonding the
receptacle layer 120 to the substrate layer 110 are described and
claimed in the copending patent applications entitled "Process for
Assembling Three-Dimensional Systems on a Chip and Structure Thus
Obtained," Serial No. 60/326,076; "Method For Assembly Of
Complementary-Shaped Receptacle Site And Device Microstructures,"
Serial No. 60/326,055, and "Method of Self-Latching for Adhesion
During Self-Assembly of Electronic or Optical Circuits," Serial No.
60/326,056. However, other processes than those described in these
copending patent applications may also be used fabricating
embodiments of the present invention.
The MMICs 121 contained in the receptacle layer 120 use
semiconductor technology appropriate for processing millimeter wave
signals. Such technologies include low noise InP
high-electron-mobility transistor (HEMT) technology and GaAs
heterojunction bipolar transistor (HBT) technology. These
technologies make it possible to directly amplify the millimeter
wave radiation at each pixel in a focal plane and to rectify these
signals. The required amplification and rectification functions may
be provided by a plurality of MMICs or these functions may be
integrated into a single MMIC.
The encapsulation layers 140 cover the MMICs 121 contained in the
receptacle layer 120 and positionally fix them in the receptacle
layer 120. The encapsulation layers 140 comprise material similar
to the material used for the receptacle layer 120, in that the
material provides a similar temperature expansion and also has good
dielectric properties. Interconnects from the MMICs 121 in the
receptacle layer 120 to the integrated circuits 111 in the
substrate layer 110 may be supported by the encapsulation layer 140
above the MMICs 121. These interconnects can be provided by the
evaporation of metal on top of the encapsulation layer 140. Vias
112 are used to make connections through the encapsulation 140 and
receptacle layers 120 to the integrated circuits located in those
layers and the interconnects located on top of the encapsulation
layer 140. The vias 112 can be constructed by stamped holes
containing vacuum deposited metal or by other techniques well-known
in the art. Exemplary processes for producing the required vias are
disclosed in the copending patent application entitled "Process for
Producing High Performance Interconnects," Serial No. 60/326,054.
Other processes known in the art for forming vias may also be
used.
Special attention must be paid to the formation of the
interconnects used for carrying RF signals, such as the
interconnects between the MMICs 121 and the antenna elements 131.
These interconnects are generally more complex than the
interconnects used for low frequency signals, such as the
interconnects between the MMICs 121 and the integrated circuits 11
in the substrate. These RF interconnects are generally either
coaxial or coplanar structures. Electromagnetic modeling is
generally required to determine ohmic and radiation losses. The RF
interconnects incorporate both shielding and grounding structures
to efficiently transition between layers. The RF interconnects may
include coaxial vias, coaxial vias with coplanar-waveguide micro
strip interlevel transitions, and coplanar-waveguide to
coplanar-waveguide interlevel transitions. The low frequency
interconnects are generally simple via interconnects structures
such as single metal lines.
Other methods can be used for connecting the MMICs 121 in the
receptacle layer 120 to the integrated circuits 111 in the
substrate layer 110. Vias 112 can be used to connect directly from
the bottom of the MMICs 121 in the receptacle layer 120 to the
integrated circuits 111 in the substrate layer 110. Such a method
of fabrication would eliminate the need for metal interconnects on
top of an encapsulation layer 140. However, direct connections
between the MMICs 121 and the integrated circuits 11 may be more
difficult to fabricate and more subject to failure. The MMIC
interconnection metal could also be placed directly on the
receptacle layer 120 and vias used to connect to the integrated
circuits 111 below. This method may also be more subject to failure
due to the effects of thermal expansion causing the connection
between the interconnection metal and the MMIC to crack.
Another embodiment of the present invention has multiple receptacle
layers 120 with each receptacle layer 120 supporting integrated
circuits 121 of the same or different semiconductor technology.
This embodiment allows additional processing to be performed on the
radio frequency signal received by each antenna element 131 while
maintaining a compact size for the focal plane array. Preferably,
additional encapsulation layers 140 are used to separate the
receptacle layers 120 and to encapsulate the integrated circuits in
the receptacle layer 120 below and to provide interconnects between
the integrated circuits 121 in the receptacle layers 120 and to the
integrated circuits 111 in the substrate. However, the receptacle
layers 120 could be stacked one above the other, where the
receptacle layer 120 above provides encapsulation for the
receptacle layer 120 beneath it.
The antenna carrying layer 130 comprises a dielectric layer 132
upon which an antenna element 131 has been printed. The antenna
element 131 couples emitted millimeter wave radiation into the
radiometer pixel. Printed antenna elements suitable for coupling
millimeter wave radiation are well known in the art. The antenna
element size would be on the order of 1/2 wavelength of the
millimeter wave emissions to be received. Suitable antenna elements
131 include a microstrip patch above a ground plane, which can
receive one of two different linear polarizations depending upon
where the feed line connects to the patch. FIG. 2 shows a ground
plane 133 provided by a metal layer deposited on an encapsulation
layer 140. A metal layer may also be deposited on another
dielectric layer located beneath the antenna carrying layer 130 to
provide a ground plane 133. The antenna carrying layer 130 would
preferably be the thickest layer in the stack of layers of the
radiometer pixel 100 and would be comprised of dielectric material
having a low dielectric constant. The antenna carrying layer 130
will generally have a thickness of approximately 25 microns,
although the thickness will vary depending upon the frequency at
which the focal plane array 10 operates and the type of antenna
structure deposited on the layer 130.
Other suitable printed antenna elements for use in the present
invention include a dipole or a spiral-type antenna. A dipole may
be less efficient than a patch due to the proximity of the
dielectric and metal layers. A spiral antenna could be used in
cases where a wider signal bandwidth or sensitivity to various
polarizations are desired.
Connections between an antenna element 131 and a MMIC 121 in the
receptacle layer 120 are made by high frequency vias 122. Such high
frequency vias can be formed by techniques well-known in the art.
Stamped holes in which metal has been vacuum deposited for
connections through the antenna carrying layer 130, the ground
plane 133, and any encapsulation layers 140 is one method for
providing the necessary connections. Other possible methods for
fabricating the required vias are described in the copending patent
application entitled "Process for Producing High Performance
Interconnects," Serial No. 60/326,054 as discussed above.
Producing a good quality millimeter wave image requires high
spatial resolution. Spatial resolution is limited by diffraction,
so the optimum spacing between detector elements in the focal plane
array is 0.5 to 1 wavelengths. Therefore each millimeter wave
detector should fit into a space that is 1 to 10 millimeters on a
side. The three dimensional structure of the present invention
illustrated in FIG. 2 easily satisfies this need. The reduced size
of each detector resulting from the vertical integration of the
present invention allows a much denser array to be formed than that
provided by prior art.
FIG. 3 shows a block diagram of the elements in a single radiometer
pixel 100 of one embodiment of the present invention to provide
detection of millimeter wave radiation. This embodiment utilizes a
simple diode detector and an analog-to-digital converter to provide
a digital representation of the millimeter wave radiation present
at a pixel. The key advantage of the present invention is that this
functionality can be provided in a single three-dimensional
structure.
As shown in FIG. 3, an antenna element 131 in the antenna carrying
layer 130 is coupled to a MMIC 121 in the receptacle layer 120. The
MMIC 121 contains a radio frequency (RF) switch 126 that switches
between the signal provided by the antenna element 131 and a
temperature reference 125. The RF switch 126 samples signals from
either the antenna element 131 or the temperature reference 125,
which is a calibrated reference load. An externally applied bias
voltage (shown applied by a chopping signal generator 116 in FIG.
3), operating at video frequencies, causes the RF switch 126 to
periodically change connections between the antenna element 131 and
the temperature reference 125. The use of a temperature stabilized
reference load is commonly used for absolute temperature
calibration of a millimeter wave detector.
The output of the radio frequency switch 126 is provided to a low
noise amplifier 127. The low noise amplifier 127 delivers amplified
RF signals to a power detector 128. The power detector 128
integrates the RF signal to produce a video output signal that is
proportional to the RF input power. A typical circuit for detecting
millimeter wave energy is further described by Lo et al. in
"Monolithic, Low-Noise, Synchronous Direct Detection Receiver For
Passive Microwave/Millimeter-Wave Radiometric Imaging Systems,"
U.S. Pat. No. 5,815,113, issued Sep. 29, 1998, incorporated herein
by reference.
Alternatively, the MMIC 121 for detecting millimeter wave energy
may itself be implemented by a three dimensional multiple layer
structure 521, as shown in FIG. 5. In FIG. 5, the RF switch 126 is
contained in one layer, the power detector 128 is contained in a
second layer, and the low noise amplifier 127 is contained in a
third layer. In the three dimensional structure 521, the RF switch
126 is preferably a very low insertion loss, metal contact RF MEMS
switch built on a GaAs substrate. Low insertion loss is an
important feature for the RF switch 126. The low noise amplifier
127 is preferably a high electron mobility transistor (HEMT) built
on a InP substrate. A low noise figure is an important feature of
the low noise amplifier 127. The power detector 128 is preferably a
quantum tunneling heterostructure that uses a nearly lattice
matched InAs/AlSb/GaSb system grown on an InAs substrate. A linear
response and circuit simplicity are important features for the
power detector 128.
As discussed above, the preferable RF switch 126 in the
three-dimensional structure 521 used for detecting millimeter wave
energy is an RF MEMS switch. Very low insertion loss RF MEMS
switches built on GaAs substrates may exhibit less than 0.2 dB
insertion loss up to 40 GHz, with switch isolation greater than 60
dB at frequencies less than 5 GHz, and greater than 25 dB at 40
GHz. Two main types of RF MEMS switches known in the art are
actuated by electrostatic forces--the metal contact series switch
and the membrane shunt switch. The series switch is considered a
true switch since it utilizes a cantilever beam that closes every
time the metal contact is made and opens when the cantilever is
released. A membrane shunt switch operates based on the position of
the electrostatically actuated membrane. When the membrane is up,
the RF signal goes through the transmission line located beneath
the membrane. When the membrane is pulled down, the switch
capacitance will couple the RF signal to ground, corresponding to a
switch open position. Although both types of switches demonstrate
excellent insertion losses (less than 0.2 dB over a very broad
frequency band from DC to millimeter wave), the series switch
provides better isolation characteristics at operating frequencies
above 5 GHz, and, therefore, is preferable for use in the three
dimensional structure 521 used for detecting millimeter wave
energy.
The low noise amplifier 127 in the three-dimensional structure 521
used for detecting millimeter wave energy preferably comprises an
HEMT built on an InP substrate. An InP HEMT two-stage low noise
amplifier that exhibits a small signal gain of 7.2 dB at 190 GHz is
known in the art. This known amplifier employs AlInAs/GaInAS/InP
HEMT devices with 0.1.times.40 .mu.m periphery and is implemented
with coplaner waveguide circuitry fabricated on an InP substrate.
Gain exceeding 10 dB has been measured from 129-157 GHz. InP HEMT
devices provide higher gain and lower noise at higher operating
frequencies as compared to other solid-state devices such as
GaAs-based FETs. These characteristics are due to the superior
electron mobility and velocity on the high indium content GaInAs
channel, along with increased carrier density in the channel due to
the large conduction band discontinuity at the AlInAs/GaInAs
heterojunction.
The power detector 128 in the three-dimensional structure 521 used
for detecting millimeter wave energy is preferably a backward
diode. A preferred diode device is closely related to a Ge Esaki
diode, but uses modern epitaxial growth technology to fabricate
precisely tailored quantum tunneling heterostructures using the
nearly lattice matched InAs/AlSb/GaSb system. This preferred diode
device provides significant advantages over other types of diodes
that may be used for RF power detection. Specifically, the
preferred diode provides a greater bandwidth than that provided by
Ge diodes, and a bandwidth as good or better than Schottky diodes.
The preferred diode has zero bias operation, which Ge diodes also
have, but Schottky diodes do not. The preferred diode has better
sensitivity, that is, greater curvature at zero bias, than other
diode types, which leads to greater dynamic range. Finally, the
preferred diode has better linearity, which is provided by careful
band engineering, to ensure ideal quadratic curvature.
Returning to FIG. 3, the video signal generated by the MMIC 121 is
coupled to a synchronous video detector 118 in the radiometer
processor integrated circuit 111, which is preferably contained in
the substrate layer. A chopping signal generator 116 controls the
synchronous video detector 118 and the radio frequency switch 126
in the MMIC 121 so that the difference in millimeter wave power
between the radiated millimeter wave image and a temperature
reference can be measured. The difference in these two values
provides the effective temperature of the image emitting millimeter
wave radiation. An integrator 114 integrates the video signal,
which is provided to an analog-to-digital converter 113 to provide
a digital video signal output.
A high quality millimeter wave image also requires high temperature
resolution and a rapid refresh rate. The time it takes to produce a
millimeter wave radiation image depends ultimately on the number of
detectors operating in parallel, and the integration time .tau.
required by each detector to measure temperature T.sub.scene with
an uncertainty .DELTA.T. High quality images typically require a
.DELTA.T resolution of at least 0.5 K, and higher resolution is
especially desired for indoor applications like security screening.
Integration time is minimized by using a receiver with a wide
bandwidth B and low noise T.sub.sys, according to the radiometer
law: ##EQU1##
High performance low noise amplifiers (LNA) provided as discrete
devices are well-known in the art. For example, 30 GHz LNAs with a
10 GHz bandwidth can provide a T.sub.sys of 170.degree. K. With
such an LNA, a typical scene at a millimeter wave temperature of
300.degree. K can be detected with a resolution .DELTA.T of
0.5.degree. K in 85 .mu.s and a resolution of .DELTA.T of
0.1.degree. K in 2.2 ms. Other LNAs with a 10 Ghz bandwidth provide
a T.sub.sys of 600.degree. K. LNAs implemented by using one or more
MMICs have similar characteristics. With an LNA with the
characteristics described above, a typical scene at 300.degree. K
can be detected with a resolution of 0.5.degree. K in 320 .mu.s and
0.1.degree. K in 8 ms. With such devices, it is possible to
generate images with exceptional 0.1.degree. K resolution at video
rates exceeding 100 frames per second, provided that each detector
element has its own video processor and the antenna-to-LNA
connection is of minimum length.
Vertical integration of radiometer pixels onto a substrate and the
small size of the pixels allows for multiple pixels to be
constructed on a substrate. One embodiment of the present invention
provides for the construction of a focal plane radiometer array
structure on a single silicon wafer. Such a wafer may be 3" (7.6
cm) to 8" (20.3 cm) in diameter. The silicon wafer is constructed
to contain multiple integrated circuits for processing millimeter
wave video signals. A receptacle layer and antenna carrying layer
are deposited on the silicon wafer substrate to create the focal
plane radiometer array. The relatively small size of the
wafer-based focal plane array would provide the capability for
handheld millimeter wave imagers. An alternative embodiment of the
present invention is provided by combining multiple wafer-based
focal plane arrays into a single array structure. This structure
would provide for a larger field of view and/or a higher resolution
image.
A millimeter wave imaging system 400 using the single wafer
embodiment of the focal plane array provided by the present
invention is illustrated in FIG. 4. The focal plane array 10 on a
single wafer is positioned behind a focusing device 20 so that a
scene 430 to be imaged is focused upon the focal plane array 10.
The focusing device 20 may be adjustable to allow a viewed image to
be enlarged or shrunken as would be provided by a zoom lens. The
focal plane array 10 receives and processes millimeter wave
radiation from the scene 430 and provides a full image video output
410 that is continuously updated. The update rate depends upon the
desired resolution, as described above. The full image video output
410 is provided to a video control and signal processor unit 420
that may perform additional processing on the millimeter wave
image. The video control and signal processor unit then transfers
the image to a video display 440. The video display produces an
image 445 based on the millimeter wave radiation detected from the
scene 430. The video display 440 may comprise a CRT or other
display devices, but, preferably, the display 440 is a liquid
crystal display so that the cost, weight and size of the millimeter
wave imaging system 400 can be kept as low as possible. A power
supply 405 is used to provide power to all the active components in
the system. Since the focal plane array 10 may be provided with a
single wafer, the entire millimeter wave imaging system 400 may be
contained in a single enclosure with a size and weight amenable for
use as a handheld unit. Multiple wafers may be used to make up the
focal plane array 10, which may increase the size of the enclosure
in which the millimeter wave imaging system 400 is contained. Use
of multiple wafers may also require that the video control and
processor unit 420 process multiple full image video outputs
410.
From the foregoing description, it will be apparent that the
present invention has a number of advantages, some of which have
been described above, and others of which are inherent in the
embodiments of the invention described herein. Also, it will be
understood that modifications can be made to the apparatus for
millimeter wave imaging described herein without departing from the
teachings of the subject matter described herein. As such, the
invention is not to be limited to the described embodiments except
as required by the appended claims.
* * * * *